The present invention relates to an electronic control circuit for a fluorescent lamp. More particularly, the present invention relates to a sensor and dimmer circuit for use with a fluorescent lamp ballast and fluorescent lamp.
Fluorescent lamps and housings therefor can be used as task lights for work station areas. See U.S. patent application Ser. No. 08/069,932 filed on May 28, 1993, entitled "Motion Sensor Assembly," by inventor Greg A. Brownell (attorney docket number 12636/80942), currently pending, the specification and drawings of which are incorporated herein by reference.
A fluorescent lamp is an electrical discharge device filled with a gas produced by vaporizing droplets of mercury within a closed tube. The inner surface of the lamp is coated with a fluorescent powder such as phosphor crystals. Electrodes (filaments) are located at each end of the tube for trigger start and rapid start lamps. When a proper voltage is applied to the filaments, they heat and ionize the gas within the tube. The ionized gas in turn causes the fluorescent powder to emit visible light. This condition within the tube is sometimes referred to as lamp arc. Lamp arc is the current flow through the ionized gas within the lamp which in turn causes the phosphor coating to fluoresce. The voltage required to produce lamp arc is often larger than the voltage supplied by conventional (e.g. 115 volts rms) electric outlets. Once lamp arc begins, the impedance of the lamp lowers. The lamp arc current is limited to a predetermined value by the impedance of a ballast winding, to provide the specified lamp wattage. The voltage across the lamp is accordingly reduced by the combination of the arc current and the ballast impedance. Current is limited in order to increase lamp life or, in some cases, prevent lamp destruction.
The ballast is used to supply a proper voltage to start the fluorescent lamp (starting voltage). The ballast supplies the starting voltage to both heat the filaments and begin lamp arc. Once lamp arc begins, the lamp voltage is reduced by the impedance characteristics of the lamp to an operating voltage. When the operating voltage is applied across the lamp, the lamp current is limited to a predetermined value by the output impedance of the ballast. The operating voltage remains across the lamp until the lamp is turned off.
In some applications, a user may desire to control (dim) the light output (i.e., brightness) of a fluorescent lamp once the lamp has started. Dimming can be accomplished in a variety of ways. One method in which fluorescent lamp dimming can be accomplished is through the use of dimming or electronic ballasts. These ballasts perform the functions of "conventional ballasts" described above in addition to allowing a user to control the brightness of the lamp. These ballasts, however, are more expensive than "conventional ballasts" and often require an external dimming control. Another method in which fluorescent lamp dimming can be accomplished is by controlling the input power to the ballast. One method which accomplishes this utilizes a phase control circuit to control the average voltage applied to the input of the ballast. Changing the phase angle of the input voltage produces harmonic currents which reduce the rated power factor of the ballast. Thus, less of the power drawn by the ballast is used to light the lamp. This results in a ballast that does not utilize power as efficiently. In addition, such phase control circuits can cause harmonic distortion of input current waveforms that add on the neutral wire of an alternating current power source such that the current carried by the neutral wire is in excess of a rated amount. A further method in which fluorescent lamp dimming can be accomplished is by electrically switching loads into a lamp circuit to divert power from the lamp. Use of such loads, however, tends to involve bulky and complicated wiring.
A fluorescent lamp dimmer that solves the problems associated with the above-described dimmer circuits would be a welcome improvement. Such a fluorescent lamp control circuit is provided by the present invention. The subject fluorescent lamp control circuit includes a dimmer and a sensor. The dimmer is electrically connected in parallel with the fluorescent lamp. The sensor is electrically connected in parallel with the dimmer and lamp. The dimmer adjustably controls the brightness of the lamp when an operating voltage is applied to the lamp. The sensor electrically disables the dimmer when a starting voltage is applied to the lamp and electrically enables the dimmer when the operating voltage is applied to the lamp.
The sensor includes a voltage controlled switch, a voltage regulator, a generally current voltage supply, and a voltage controlled trigger. An output of the voltage regulator is electrically connected to an input of the voltage controlled switch. The voltage regulator supplies a voltage to the voltage controlled switch to open and close the switch. The voltage controlled trigger is electrically connected between an output of the direct current voltage supply and an input of the voltage regulator. When triggered, the voltage controlled trigger supplies current to the voltage regulator.
In a preferred embodiment, the voltage controlled switch of the sensor includes inverse-series metal-oxide semiconductor field-effect transistors (MOSFETs). In this embodiment, the output of the voltage regulator is connected to the gates of the inverse-series metal-oxide semiconductor field-effect transistors. The voltage regulator may include a capacitor electrically connected in parallel with a zener diode and a resistor. The direct current voltage supply may include a half-wave rectifier that can be constructed from a diode which is electrically connected in series with a capacitor. The direct current voltage supply may further include a voltage divider electrically connected in parallel with the capacitor of the half-wave rectifier. This voltage divider provides at least two voltages, one of which appears at the output of the direct current voltage supply. A filter is electrically connected to the voltage appearing at the output of the direct current voltage supply. The filter enhances linearization of the voltage to which the filter is electrically connected. The filter may include a capacitor electrically connected in parallel with the output of the direct current voltage supply. The trigger may include a sidac.
The dimmer includes a current controlled switch, a variable phase-shift (RC) network, and a voltage controlled switch. The voltage controlled switch is electrically connected between an output of the variable phase-shift (RC) net and an input of the current controlled switch.
In a preferred embodiment, the current controlled switch includes a triac. A snubber circuit is electrically connected in parallel with the triac. The snubber circuit suppresses the sudden voltage rises appearing across the triac caused by the ballast when the triac opens. The snubber circuit may include a capacitor and a resistor. The variable phase-shift (RC) network may include a voltage divider network connected in series with a capacitor. The voltage divider network includes a device that allows the triggering of the voltage controlled switch to be adjusted at a particular phase angle of the operating voltage. The adjusting device may include a potentiometer. The variable phase-shift (RC) network may also include an anti-hysteresis network. The anti-hysteresis network may include a resistor and a capacitor. The voltage controlled switch includes a diac.
The dimmer may also include a radio frequency interference filter for suppressing radio frequency interference caused by the dimmer. The radio frequency interference filter may include an inductor that is electrically connected with a capacitor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.